1. Field of the Invention
The present invention relates to a photoreceptor (image bearing member) and an image forming method, an image forming apparatus, and a process cartridge using the photoreceptor.
2. Description the Background Art
Organic photoconductors (OPC) (photoreceptors) have good characteristics and have been used in place of inorganic photoreceptors in photocopiers, facsimile machines, laser printers, and multi-functional devices thereof in light of various advantages. Specific reasons for this supersession include: (1) good optical characteristics, for example, a broad range of optical absorption wavelengths and a large amount of light absorption; (2) superior electrical characteristics, for example, high sensitivity and stable chargeability; (3) a wide selection of materials; (4) ease of manufacturing; (5) inexpensive cost; and (6) non-toxicity.
In addition, in an attempt to manufacture a compact image forming apparatus, the diameter of the photoreceptor is constantly being reduced. Furthermore, with the advancement toward higher speed performance and maintenance-free machines, a photoreceptor having excellent durability has been desired. From this point of view, an organic photoconductor is soft in general and wears out easily because the charge transport layer of the organic photoconductor is mainly made of a low molecular weight charge transport material and an inert polymer. Thus, the organic photoconductor repetitively used in the electrophotographic process tends to be easily abraded under the mechanical stress by a developing system or a cleaning system.
Moreover, to meet with the demand for high-quality images, toners consisting essentially of smaller toner particles have been used recently. The smaller particles require better cleaning performance, which leads to inevitable usage of a harder cleaning blade and an increase in the contact pressure between the cleaning blade and the photoreceptor. This is another factor accelerating the abrasion of the photoreceptor.
Such abrasion of the photoreceptor causes deterioration of electrical characteristics such as the sensitivity and the chargeability, resulting in production of defective images having, for example, a low image density and background fouling. Localized damage to the photoreceptor due to abrasion causes production of defective images with streaks ascribable to bad cleaning of the photoreceptor.
A number of attempts have been made to improve the abrasion resistance of the organic photoconductor. For example, Japanese Patent Application Publication No. S56-048637 (JP-S56-048637-A) describes a photoreceptor using a curable binder resin in the charge transport layer; JP-S64-001728-A describes a photoreceptor using a charge transport polymer; JP-H04-281461-A describes a photoreceptor having a charge transport layer in which inorganic fillers are dispersed; Japanese Patent No. 3262488 (JP-3262488-B) describes a photoreceptor containing multi-functional acrylate monomer cured materials; JP-3194392-B describes a photoreceptor having a charge transport layer formed by a liquid application formed of a monomer having a carbon-carbon double bonding, a charge transport material having a carbon-carbon double bonding, and a binder resin; JP-2000-66425-A describes a photoreceptor containing a compound formed by curing a positive hole transport compound having two or more chainable polymerizable functional groups in a single molecular; JP-H06-118681-A describes a photoreceptor using a curable silicon resin containing colloidal silica; JP-H09-124943 and JP-H09-190004 describe photoreceptors having a resin layer formed by binding an organic silicon modified positive hole transport compound in a curable organic silicon-based polymer; JP-2000-171990-A describes a photoreceptor formed by curing a curable siloxane resin having a charge transport property imparting group to have a three dimensional network structure; JP-2003-186223-A describes a photoreceptor containing a binder resin cross-linked in three dimensions with a charge transport material having one or more hydroxyl group and electroconductive particulates; JP-2007-293197-A describes a photoreceptor containing a cross-linked resin formed by cross-linking a reactive charge transport material, a polyol having two or more hydroxyl groups, and an aromatic isocyanate compound; JP-2008-299327-A describes a photoreceptor containing a melamine formaldehyde resin which is three-dimensional cross-linked with a charge transport material having one or more hydroxyl group; and JP-4262061-B describes a photoreceptor containing a resol-type phenolic resin which is three-dimensional cross-linked with a charge transport material having a hydroxyl group.
Furthermore, JP-2006-251771-A describes a photoreceptor containing an optically functional organic compound that can form a cured film, sulfonic acid and/or its derivative, and an amine having a boiling point of 250° C. or lower; and JP-2009-229549-A describes a photoreceptor formed of a cross-linked material prepared by a liquid application that contains either or both of a guanamine compound and a melamine compound and at least one charge transport material having one or more substitution groups selected from the group consisting of —OH, —OCH3, —NH2, —SH, and —COOH, where the concentration of the solid portion of the material selected from the guanamine compound and the melamine compound in the liquid application ranges from 0.1% by weight to 5% by weight and the concentration of the solid portion of the at least one charge transport material in the liquid application is 90% by weight or more.
Among these photoreceptors, those having a three-dimensional cross-linked surface layer have excellent mechanical durability, which is advantageous in prolonging the working life of the photoreceptors. For example, the photoreceptor described in JP-2000-66425-A mentioned above can have a three-dimensional cross-linked layer formed by radical polymerization using ultraviolet light or an electron beam and demonstrates an excellent abrasion resistance. However, such a photoreceptor accompanies problems such that large-scale instrumentation to emit the ultraviolet light or the electron beams, thereby degrading the productivity. Moreover, the charge transport material tends to deteriorate due to the irradiation by these, which results in deterioration of the voltage characteristics of the photoreceptor. By contrast, the photoreceptors described in JP-2007-293197-A, JP-2008-299327-A, JP-4262061-B, JP-2006-251771-A, and JP-2009-229549 mentioned above are easy to produce because the three-dimensional layer can be formed by thermosetting, and moreover they demonstrate excellent abrasion resistance.
However, since the photoreceptor described in JP-2007-293197-A mentioned above is cured by urethane bonding, the charge transport property is inferior and the photoreceptor exhibits poor electrical characteristics. The photoreceptors described in JP-2008-299327-A, JP-4262061-B, JP-2006-251771-A, and JP-2009-229549-A mentioned above have a surface layer formed by three-dimensional cross-linking the charge transport compound having a polar group such as a hydroxyl group and a reactive active species such as melamine and demonstrates relatively excellent electrical characteristics. In particular, the photoreceptor described in JP-2009-229549-A mentioned above demonstrates excellent charge transportability and good sensitivity because it uses a charge transport compound with a high ratio of 90% or more by weight. However, the polar groups such as OH group contained in the charge transport compound inevitably remains in the three dimensionally cross-linked layer, thereby reducing the charge. As a result, the image density tends to decrease in a high temperature, high humidity environments, as well as with exposure to NOx gas, etc, produced by a charger.
JP-2006-84711-A describes a photoreceptor formed by curing a compound in which OH groups, etc. in a charge transport compound are blocked with a reactive activated species such as a melamine. In this case, although it is possible to prevent the polar group from remaining in the cross-linked layer, the blocked OH group and the reactive activated species tend not to react easily. Therefore, forming a three-dimensional cross-linked layer having excellent mechanical characteristics is difficult.
As described above, photoreceptors having a three-dimensional cross-linked layer on the surface have been investigated and prove to be effective in improving the abrasion resistance.
On the other hand, there is a defect of the three dimensional cross-linked layer, which is that the charge transport property deteriorates because the charge transport compound is fixed in a cured matrix. As in JP-2009-229549-A mentioned above, if simply a charge transport compound having a high polar group such as OH group is used in an amount of 90% or more, the remaining amount of the charge transport compound having non-reacted OH group tends to increase, which makes it difficult to reduce the electrical potential variation caused by the operating environment.
Therefore, it is not possible to have a good combination of charge transport and the stability (for environment, gases, etc,).
Moreover, a different problem arises if a three dimensional cross-linked layer in which a charge transport compound is molecule-dispersed is used.
Once the surface of a photoreceptor having an improved abrasion resistance is damaged and/or foreign objects are firmly attached thereto, such a state continues to remain, resulting in continued production of defective images.
In particular, to deal with the demand for improvement in the quality of images and energy-saving, smaller toner particles having a lower softening temperature have been used in recent years. With regard to such toner, inorganic particulates such as silica are used to secure the fluidity of the toner.
Such silica particulates easily stick to the surface of the organic photoconductor in the development process and wax components etc., of the toner accumulate around the stuck silica particulates, which causes production of defective images having white spots.
The three dimensionally cross-linked layer in which a charge transport component having no cross-linking reactivity is molecule-dispersed tends to be phase-separated during cross-linking reaction. This easily leads to occurrence of image defects such as white spots and prevents attaining a good combination of the charge transport property and the inherent abrasion resistance of the three-dimensional cross-linked layer.
Therefore, a photoreceptor has not been provided which is highly durable and stable while maintaining inherent abrasion resistance of a three-dimensional cross-linked layer to prevent production of detective images such as white spots over repetitive use and reduce the electrical potential variation in operating environments and an image forming method, an image forming apparatus, and a process cartridge for image forming which use the photoreceptor have not been provided, either.
As the photoreceptor that stably outputs quality images in any operating environment, it is necessary to have excellent mechanical characteristics (such as abrasion resistance and damage resistance) to reduce the electrical potential variation caused by the layer thickness variation, excellent charge transport property, and excellent stability to reduce the electrical potential variation in an operating environment.